log: update windows redirection code to use x/sys/windows and include dup

This modernises the Windows redirect code to use x/sys/windows and to
dup os.Stderr for use in the terminal.

fs/log/redirect_stderr_windows.go

@@ -12,29 +12,43 @@ package log
 import (
 	"log"
 	"os"
-	"syscall"
+
+	"github.com/rclone/rclone/lib/terminal"
+	"golang.org/x/sys/windows"
 )
 
-var (
-	kernel32         = syscall.MustLoadDLL("kernel32.dll")
-	procSetStdHandle = kernel32.MustFindProc("SetStdHandle")
-)
-
-func setStdHandle(stdhandle int32, handle syscall.Handle) error {
-	r0, _, e1 := syscall.Syscall(procSetStdHandle.Addr(), 2, uintptr(stdhandle), uintptr(handle), 0)
-	if r0 == 0 {
-		if e1 != 0 {
-			return error(e1)
-		}
-		return syscall.EINVAL
+// dup oldfd creating a functional copy as newfd
+// conceptually the same as the unix `dup()` function
+func dup(oldfd uintptr) (newfd uintptr, err error) {
+	var (
+		newfdHandle   windows.Handle
+		processHandle = windows.CurrentProcess()
+	)
+	err = windows.DuplicateHandle(
+		processHandle,                 // hSourceProcessHandle
+		windows.Handle(oldfd),         // hSourceHandle
+		processHandle,                 // hTargetProcessHandle
+		&newfdHandle,                  // lpTargetHandle
+		0,                             // dwDesiredAccess
+		true,                          // bInheritHandle
+		windows.DUPLICATE_SAME_ACCESS, // dwOptions
+	)
+	if err != nil {
+		return 0, err
 	}
-	return nil
+	return uintptr(newfdHandle), nil
 }
 
 // redirectStderr to the file passed in
 func redirectStderr(f *os.File) {
-	err := setStdHandle(syscall.STD_ERROR_HANDLE, syscall.Handle(f.Fd()))
+	termFd, err := dup(os.Stderr.Fd())
+	if err != nil {
+		log.Fatalf("Failed to duplicate stderr: %v", err)
+	}
+	terminal.RawOut = os.NewFile(termFd, "termOut")
+	err = windows.SetStdHandle(windows.STD_ERROR_HANDLE, windows.Handle(f.Fd()))
 	if err != nil {
 		log.Fatalf("Failed to redirect stderr to file: %v", err)
 	}
+	os.Stderr = f
 }

lib/terminal/terminal.go

@@ -81,8 +81,11 @@ func Start() {
 			// If output is not a tty then remove escape codes
 			Out = colorable.NewNonColorable(f)
 		} else if runtime.GOOS == "windows" && os.Getenv("TERM") != "" {
-			// If TERM is set just use stdout
-			Out = os.Stdout
+			// If TERM is set on Windows then we should just send output
+			// straight to the terminal for cygwin/git bash environments.
+			// We don't want to use NewColorable here because it will
+			// use Windows console calls which cygwin/git bash don't support.
+			Out = f
 		} else {
 			Out = colorable.NewColorable(f)
 		}